After trying the dual navigation layout under Window->Layouts I can no longer open documents in the right editor, even when it’s selected.
I figured it out. It looks like using that layout changes an option I didn’t know existed; Right click an item in Binder and there’s an option named Binder Selection Affects. I just had to change this to Both Editors.
I feel like that should have been done automatically when I went to Window->Layouts->Default.